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The Beta Particle

Beta decay is a radioactive process in which an electron is
emitted from the nucleus of a radioactive atom, along with an
unusual particle called an antineutrino. The neutrino is an
almost massless particle that carries away some of the energy
from the decay process. Because this electron is from the nucleus
of the atom, it is called a beta particle to distinguish it from
the electrons which orbit the atom.

Like alpha decay, beta decay occurs in isotopes which are
neutron rich (i.e. have a lot more neutrons in their
nucleus than they do protons). Atoms which undergo beta decay are
located below the line of stable elements on the chart of the
nuclides, and are typically produced in nuclear reactors. When a
nucleus ejects a beta particle, one of the neutrons in the
nucleus is transformed into a proton. Since the number of protons
in the nucleus has changed, a new daughter atom is formed which
has one less neutron but one more proton than the parent. For
example, when rhenium-187 decays (which has a Z of 75) by beta
decay, osmium-187 is created (which has a Z of 76). Beta
particles have a single negative charge and weigh only a small
fraction of a neutron or proton. As a result, beta particles
interact less readily with material than alpha particles.
Depending on the beta particles energy (which depends on the
radioactive atom), beta particles will travel up to several
meters in air, and are stopped by thin layers of metal or
plastic.

High energy betas that travel through water sometimes produce Cerenkov Radiation, which in turn produces the blue glow seen around fuel and reactors.